In an NC machining tool, a workpiece placed on a table is machined by moving a working tool, mounted to a spindle, and the workpiece relative to each other in accordance with a desirable machining program, whereby a shape of the workpiece obtained by the machining is determined by the trajectory of the working tool. Generally, the machining program is prepared on the premise that the working tool is not worn and maintains a constant length during the machining.
However, in practice, as the machining operation progresses, the working tool wears to shorten its length. Accordingly, as shown, for example, in FIG. 5, when the desired profile 41 of the workpiece is slanted upwards from left to right, the actually machined profile (i.e., machining trajectory) 43 is gradually shifted upward (i.e., toward the spindle) from the desired profile (i.e., machining trajectory) 41 to result in the deviation. Accordingly, the desirable machined profile 41 cannot be obtained.
Further, when the working tool is worn, the working tool is replaced with a new one by detecting the working tool life in a manner as described, for example, in Japanese Unexamined Patent Publication Nos. 54-35485 or 2001-150299. That is, the machining operation is interrupted and, after the worn working tool has been replaced with the new one having a standard length, the machining operation starts again. However, in such a case, if the new working tool is returned to a command position in accordance with the machining program, an actual position of the edge of the new working tool is shifted to lower than that of the old working tool by the wear amount thereof because the new working tool is not yet worn. Similarly, in the case that a working tool which is being used for machining a first area interrupts its machining operation and starts again after carrying out the machining of a second area different from the first area, the wear amount of the working tool after starting again the machining operation in the first area has changed from that before interrupting the machining operation in the first area. Therefore, when the working tool is restored to the command position at which the machining operation in the first area has been interrupted, the actual position of the edge of the working tool after starting again the machining operation in the first area changes from that before interrupting the machining operation in the first area. As a result, there is an inconvenience in that the level difference 45 (see FIG. 5) is generated on the machined profile between before and after the replacement of the working tool or the interruption of the machining operation in the purpose of machining different areas. In a case shown in FIG. 5, as the working tool is replaced three times, there are three level differences 45 in the machined profile.